1. Field of the Invention
The invention relates to an X-ray examination apparatus which includes an X-ray source, an X-ray detector, and an exposure control system for controlling the X-ray source and the X-ray detector, which exposure control system is arranged to control the X-ray source so as to carry out a test exposure with a low X-ray dose while producing a control signal by the X-ray detector, and to control the X-ray source on the basis of the control signal in order to carry out an X-ray exposure with a high X-ray dose and to acquire an X-ray image by the X-ray detector during this exposure.
2. Description of the Related Art
An X-ray examination apparatus known in the art is German Offenlegungsschrift DE 43 30 787.
An object to be examined during a radiological examination, for example a patient to be examined, is exposed to an X-ray dose in order to form one or more X-ray images of the object. The X-ray dose required for individual patients to be examined is customarily adjusted individually. The X-ray dose is adjusted by the exposure control system.
The known X-ray examination apparatus carries out the test exposure first with a comparatively low dose, thus forming a test image. During the formation of this test image, the patient to be examined is briefly exposed to X-rays with a predetermined intensity and energy. Because X-rays are applied only during a short period of time during the formation of the test image, the X-ray dose for forming the test image remains comparatively small. The test image is read out from the X-ray detector and digitized in digital grey scale values with, for example, a bit depth of 10 bits. The exposure control system of the known X-ray examination apparatus derives the X-ray dose which is required to form the X-ray image during the X-ray exposure from the distribution of the digital grey scale values. The X-ray dose required for the formation of the X-ray image is large in comparison with the X-ray dose required to form the test image. In the known X-ray examination apparatus, the larger X-ray dose is obtained by using a longer exposure time with the same intensity and energy of the X-rays as used during the test exposure. The known X-ray detector is provided with a sensor matrix having a large number of sensor elements.
It is a drawback of the known X-ray examination apparatus that the reading out of the X-ray detector after the test exposure requires the same amount of time as the reading out after the X-ray exposure whereby the X-ray image is formed. Consequently, the adjustment of the X-ray source for the X-ray exposure requires a comparatively long period of time.